Shiites react during the Muslim festival of Arbaeen at the shrine of Imam Hussein in Karbala, south of Baghdad.

Photo: Hadi Mizban, Associated Press

Shiites react during the Muslim festival of Arbaeen at the shrine...

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Iranian mourners attend a ceremony commemorating Arbaeen, the final day of the annual 40-day mourning period for the seventh century martyrdom of Imam Hussein, the grandson of Prophet Muhammad, in Tehran, Iran, Thursday, Jan. 3, 2012. (AP Photo/Vahid Salemi)

A car bomb explosion tore through a crowd of Shiite pilgrims returning home Thursday from a religious commemoration, killing at least 20 and reinforcing fears of renewed sectarian violence, according to Iraqi officials.

The blast erupted late in the afternoon in the town of Musayyib, about 40 miles south of the Iraqi capital. It targeted worshipers returning from the Shiite holy city of Karbala following the climax of the religious commemoration known as Arbaeen.

Children were among the 20 people confirmed killed, according to a police official. He said at least 50 people were wounded.

The bomb went off in the middle of a gathering of pilgrims changing buses coming from Karbala on their way to other destinations in the country, according to police.

"The explosion shook the whole block and smashed the windows of my house," said teacher Ibrahim Mohammed, who lives nearby. "I ran to the scene of the explosion only to find charred bodies and burning cars. There were women screaming and searching for their missing children."

Thursday marked the height of Arbaeen, when hundreds of thousands of Shiite pilgrims converged on Karbala to mark the passing of 40 days after the anniversary of the seventh century martyrdom of the revered Shiite saint Imam Hussein, the grandson of the prophet Muhammad.

Shiite pilgrims are one of the favorite targets for Sunni insurgents during Shiite religious events.

Iraqi authorities typically tighten security in Karbala and along routes used by pilgrims, but security forces acknowledge they are unable to prevent all attacks.

As in previous years, the pilgrims practiced the ritual of self-flagellation on the streets, hoisted Shiite religious flags on trees and lampposts and served food from tents pitched on street corners.

State television earlier Thursday aired video of Prime Minister Nouri al-Maliki walking among the pilgrims.

Thursday's attack came after Iraqi authorities ordered the release of 11 Sunni women facing criminal charges and pledged to transfer other women prisoners to jails in their home provinces.

The move appeared aimed at addressing a main demand during a wave of protests by the country's Sunni minority against the Baghdad government.

The demonstrations erupted after the arrest of bodyguards assigned to Finance Minister Rafia al-Issawi, one of the central government's most senior Sunni officials. The protests tap into deeper Sunni feelings of perceived discrimination and unfair application of laws against their sect by Shiite Prime Minister al-Maliki's government.

Vice President Tariq al-Hashimi, another high-ranking Sunni official, is living in exile in Turkey after he was handed multiple death sentences in absentia for allegedly running death squads, a charge he dismisses as politically motivated.